Many impact type tools having torsion or rotation speed adjustment are available in the market and are used to achieve engagement between a fastener and a workpiece. Generally, when used in a fastening operation, an impact type fastening tool will have an impact fastening phase which follows a screwing phase. During a fastening operation application of a too large torsion will damage the fastener at the impact phase and application of a too small torsion will not allow completion of the fastening operation, so that the fastener and the workpiece get loose. It is for this reason that current impact type fastening tools are generally provided with several torsions or rotation speed control positions. Such control positions are generally used to drive a motor with different duty cycles, and is characterized by high rotation speed with high torsion and low rotation speed with low torsion, that is to say, a higher duty cycle will result in a higher rotation speed and torsion.
FIG. 1 illustrates a relationship diagram showing a change over time of rotation speeds (and accordingly torsion) at different rotation speed control positions of current impact type fastening tools. As shown in FIG. 1, each rotation speed control position comprises a stage in which the rotation speed (and accordingly torsion) rises slowly along with the current of the motor during an initial start period. After a designated position is reached, the rotation speed is output constantly at a constant value set according to the rotation speed control position.
In actual application, there usually occurs such a problem that a user selects a control position of rotation speed or torsion to obtain a suitable torsion at the impact phase as shown in FIG. 1, and the impact type fastening tool will achieve the driving according to the rotation speed. To prevent the fastener from being damaged due to too large a torsion, the user sets a safe and relatively low torsion, and a corresponding rotation speed is a relatively low rotation speed. However, such rotation speed and torsion waste efficiency for the screwing phase.
Currently, there is not yet a rotation control method for allowing for efficient rotation before entry into the impact phase and allowing for a switch to a suitable rotation speed to obtain a suitable torsion after automatic judgment of entry into the impact phase.